In heat exchangers such as aluminum condensers for automobiles, as generally shown in FIG. 8, a flat heat exchanging tube 200, which is called as a harmonica tube having a number of small hollow portions 202 arranged in a width direction thereof via partitioning walls 201, is used.
In manufacturing such a multi-hollowed tube by extrusion forming, an extrusion die having a female die for defining an outer periphery of the tube and a mandrel 300, as shown in FIG. 11, to be combined with the female die, is used.
The mandrel 300 has a body 301 and a comb-shaped portion protruded from the body 301 having a plurality of columnar portions 302 and arranged in the width direction of the comb-shaped portion at certain intervals. The outer periphery of the tip end portion of each columnar portion 302 constitutes a bearing portion 303 for defining an inner periphery of each of plural hollow portions arranged in the width direction of the tube.
This mandrel 300 is combined with the female die such that the bearing portions 303 of the comb-shaped portion are disposed in the opening of the female die to constitute an extrusion die. In manufacturing the extruded tube, extrusion material such as an aluminum billet loaded in a container is pressurized by a stem and the like from the rear side of the mandrel 300 to forcibly introduce the extrusion material into a gap between the female die and the columnar portions 302 and a gap between adjacent columnar portions 302, to thereby extrude the material continuously into the extrusion tube.
Now, as a heat exchanger is required to be small in size and high in performance, a multi-hollowed extrusion tube 200 for use in heat exchangers is also required to reduce the thickness of the partitioning wall 201 and the width of each hollow portion 202. In order to attain the requirements, the mandrel 300 of the die is also required to further reduce the gap 304 between the bearing portions 303 of the adjacent columnar portions 302 and/or the width of the bearing portion of each columnar portion 302.
In the mandrel 300, however, if the gap 304 between the bearing portions 303 of the adjacent columnar portions 302 is decreased, the extrusion material cannot be introduced into the gap 304 fully, causing defects of the partitioning wall 201 of the tube 200.
In order to avoid the aforementioned problem, as shown in FIG. 11, in some extrusion dies, a gap 305 between basal end portions of the adjacent columnar portions 302 is enlarged so that the gap becomes larger than the gap between the adjacent bearing portions 303 so that the extrusion material can be introduced into the enlarged gap 305 and therefore sufficient extrusion material can be supplied to the gap 304 between the adjacent bearing portion 303 to thereby prevent generation of defects due to insufficient extrusion material.
In cases where the number of small hollow portions of the tube exceeds a certain number, if the width of the bearing portion 303 of the columnar portion 302 is decreased in order to decrease the width of each hollow portion 202 of the tube 200, the thickness of the columnar portion 302 forming the enlarged gap 305 becomes thinner than the gap 304 of the bearing portion 303, causing decreased strength of the columnar portion 302, which results in an easy-to-break columnar portion. Thus, there is a limit to decrease the width of each hollow portion 202 of the tube 200.